The invention relates to apparatus for coating a fiber, and more particularly for coating a glass optical fiber with resin.
Depositing a concentric protective coating on an optical fiber that is produced by high speed drawing, i.e. drawing at a speed in excess of 10 meters per second, requires the use of an injector that enables the coating resin to be distributed uniformly, so as to ensure that defects that could degrade the quality of the fiber, and in particular coating eccentricities, are limited as much as possible.
In conventional manner, an optical fiber is coated after a fiber-drawing operation performed on a preform from which the fiber is drawn. The fiber then passes through two dies that are in vertical alignment and through a resin injector that is placed between the dies. The injector has a grid of tubular appearance with the fiber for coating passing along the axis thereof. The inside of the grid is filled with coating resin which is deposited as a layer on the fiber as it passes through the grid. The inside volume defined by the grid is filled with resin under pressure from the outside and through holes made in the cylindrical wall of the grid. The fiber travelling at high speed through the resin that is present inside the grid of an injector causes the resin to be set into motion that gives rise to a high degree of circulation of the resin inside the grid. This is due to the fact that there exists a speed field of very high gradient within the resin inside the grid when the travel speed of the fiber on the axis of the grid is high, and given that the travel speed of the resin at the cylindrical wall is practically zero, when using grid tubes of the diameters usually provided.
Experience shows that with the grids normally used, the eccentricity of the coating on the fiber increases with increasing travel speed of the fiber. Given that the flow rate of the coating fiber increases with increasing speed of the fiber, one possibility is that the inlet speed and the direction of the resin through the grid of the injector have an influence on eccentricity, with eccentricity tending to increase with increasing inlet speed.
This gives rise to high levels of head loss in the resin as it enters the tube of the grid through the cylindrical wall of the tube. It is therefore advantageous to reduce such head loss to as small a value as possible. There also exist non-uniformities in resin inlet speed distribution through the gird that can lead to phenomena that give rise to eccentricity appearing. In addition, high rates of shear can exist towards the bottom of the volume of resin contained inside the grid, and more precisely at the entrance to the finishing die through which the coated fiber is delivered.
It would also be advantageous to balance the resin flow so as to avoid coating defects of the gap type or of the bead type. It is known that good balance gives rise to a meniscus-like equilibrium surface coming into existence that is maintained by surface tension where the fiber penetrates into the resin contained in the grid. It is therefore important for the coating apparatus, and in particular the injector, to be organized in such a manner as to facilitate maintaining meniscus equilibrium while coating is taking place.
The invention thus proposes fiber coating apparatus comprising an injector provided with a tubular grid that is mounted between two dies between which the fiber travels vertically on the axis of the grid, the grid having a wall of cylindrical appearance and being filled with a coating resin which penetrates therein via distributed holes through its wall.
According to a characteristic of the invention, the apparatus has an injector grid whose cylindrical wall of outside diameter dext is situated at a minimum distance from the wall of diameter Dext that surrounds it in the injector, said distance being such that the ratio Dext/dext is not less than 1.35, and the difference Dextxe2x88x92dext is greater than or at least equal to 4 millimeters. The regularity distributed holes through said walls have a minimum diameter of not less than 0.50 mm and a density per unit area of not less than 0.20 holes per mm2. The height of the grid lies in the range 5 mm to 50 mm, and the difference between the outside and inside diameters of the grid wall dextxe2x88x92dint is not greater than 10 mm, with a minimum inside diameter dint of 5 mm.